Skin cancer is the most common malignancy with more than 1 million new cases diagnosed annually in the United States alone. Solar UVB (ultraviolet B) radiation is the main etiologic factor for skin cancer; primarily, it causes DNA damages in skin epidermal cells which, if un-repaired, potentially lead to initiated cells. These initiated cells harbor mutations primarily in p53 tumor suppressor gene that eventually lead to their clonal expansion and formation of skin tumors. Sunscreens are used to protect against solar UVB radiation; however, due to high incidence of skin cancer, additional efforts continue for developing effective agents that could act against UVB-caused damages in normal skin, eventually leading to skin cancer chemoprevention. Our completed studies suggest that silibinin is one such agent that suppresses UVB-caused skin damages and prevents skin cancer in mouse model. We have also found that silibinin lowers UVB-caused DNA damage in terms of cyclobutane pyrimidine dimmer (CPD) positive cells and further enhances p53 protein level in mouse skin epidermal cell, and selectively increases E2F1 protein level in chronic UVB exposed mouse skin. Silibinin also enhances phosphorylation and accumulation of p53 in mouse keratinocyte JB6 cells in response to UVB. Our preliminary studies in p53-/- and wild-type SKH-1 hairless mice showed a decrease in UVB-induced CPD formation in skin by silibinin through enhanced DNA repair in a p53-dependent manner. Based on these completed novel and innovative studies, which have direct relevance to human skin cancer chemoprevention by silibinin, the major focus of this application is to further identify and establish molecular targets of silibinin efficacy in preventing skin cancer. The central hypothesis proposed is that silibinin reduces UVB-caused DNA damage by enhancing DNA repair via p53 and E2F1 resulting in chemoprevention of non-melanoma skin cancer. Specific aims are: I) to define and establish the role of p53 and E2F1 in silibinin-mediated epidermal DNA repair and prevention of skin carcinogenesis in response to UVB by using genetic approaches; II) to define and establish the role of upstream signal/s in p53- and E2F1-mediated DNA repair mechanisms of silibinin in response to UVB; and III) to examine and define the role of p53 and E2F1 in nucleotide excision repair-mediated DNA repair by silibinin in response to UVB. Human Health Relevance: The present proposal is highly relevant in establishing the preventive efficacy of silibinin against UVB-induced photodamage and photocarcinogenesis with highly novel and targeted mechanistic details. Overall, since solar radiation is the most common and un- avoidable source of skin cancer, we believe that the outcomes of the present proposal will benefit each and every human being 'under the sun'. PUBLIC HEALTH RELEVANCE: Chemoprevention is the most effective measure against skin cancer, which is the single largest cause of cancer-related incidences in the United States. UVB (ultraviolet B) radiation from the sunlight is the main etiologic factor for skin cancer; primarily, it causes DNA damages in skin epidermal cells which, if un-repaired, eventually lead to their clonal expansion and formation of skin tumors. Sunscreens have partial protective effect against UVB radiation; however, due to high incidence of skin cancer, the efforts continue for an effective agent that could act against UVB-caused damages in normal skin, eventually leading to skin cancer chemoprevention. Our completed studies suggest that silibinin is one such agent that suppresses UVB- caused skin damages and prevents non-melanoma skin cancer (NMSC). Silibinin is already consumed extensively in the United States and world-wide as a dietary supplement and used in the clinic as a hepatoprotective agent. This compound has not shown any toxicity in animal studies as well as in humans. The outcomes of our proposed research will establish preventive efficacy of silibinin against UVB-induced photodamage and photocarcinogenesis, and will build the foundation for future chemoprevention clinical trial with silibinin against non-melanoma skin cancer growth and development in humans.